1. Field of the Invention
The present invention relates to an image forming optical system and an electronic image pickup apparatus using the same.
2. Description of the Related Art
In recent years, there has been a progress in achieving high magnification and slimming of a compact camera. For achieving high magnification, it is necessary to make strong a refractive power of each lens group, and to increase an amount of movement at the time of zooming. However, when the refractive power of each lens group is made strong or when the amount of movement at the time of zooming is increased, an aberration and an increase in a length of an optical system are susceptible to occur.
Meanwhile, in a compact camera, when the camera is not being used for capturing an image, in a large number of cases, an optical system is made to collapse and is accommodated inside a camera casing, and for slimming of the camera, it is necessary to reduce a thickness of the optical system when collapsed. Here, for making the optical system collapse, a lens frame of the optical system is to be let to expand and contract by dividing into a plurality of stages. At this time, when the thickness of the optical system when collapsed becomes thin, a length of each lens frame which has been divided is to be shortened. Consequently, the number of stages at the time of making the optical system collapse increases. Accordingly, when the optical system is elongated to be longer, the overall lens frame is susceptible to deform due to gravitational force, and decentering of the optical system is susceptible to occur. Therefore, for reducing the number of stages to a small number while shortening an interval of dividing the lens frame, it is necessary to shorten the overall length of the optical system.
For fulfilling these requirements, it is preferable to shorten the overall length of the optical system while achieving high magnification without allowing degradation of an image forming performance. As an example of such optical system, an optical system described in Japanese Patent Application Laid-open Publication No. Hei 8-68941, is available. The optical system described in Japanese Patent Application Laid-open Publication No. Hei 8-68941, is an optical system which includes in order from an object side a lens group having a positive refractive power, a lens group having a negative refractive power, a lens group having a negative refractive power, a lens group having a positive refractive power, and a lens group having a positive refractive power. By making an arrangement such that there are two lens groups having a negative refractive power, and that a refractive power of the negative lens groups is formed to be weak, a high-magnification image forming optical system having a superior optical performance while shortening the overall length of lenses has been realized.